Embodiments of the present invention relate generally to turbine components, and more particularly to apparatus and methods for constructing single-crystal turbine components for use in high-temperature environments
A typical gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine includes one or more stages which extract energy from the primary gas flow. Each stage comprises a stationary turbine nozzle followed by a downstream rotor carrying turbine blades. These “hot section” components operate in an extremely high temperature environment which promotes hot corrosion and oxidation of metal alloys.
In the prior art, hot section components are typically cast from nickel- or cobalt-based alloys having good high-temperature creep resistance, known conventionally as “superalloys.” These alloys are primarily designed to meet mechanical property requirements such as creep rupture and fatigue strengths.
The casting process is known to produce desired microstructures, for example directionally solidified (“DS”) or single-crystal (“SX”). A single-crystal microstructure refers to a structure which is free from crystallographic grain boundaries. Single crystal casting requires a seed element (that is, a nucleation point for cooling) and careful control of temperatures during cooling.
Additive manufacturing is an alternative process to casting, in which material is built up layer-by-layer to form a component. Unlike casting processes, additive manufacturing is limited only by the position resolution of the machine and not limited by requirements for providing draft angles, avoiding overhangs, etc. as required by casting. Additive manufacturing is also referred to by terms such as “layered manufacturing,” “reverse machining,” “direct metal laser melting” (DMLM), and “3-D printing.” Such terms are treated as synonyms for purposes of the present invention.
Prior art techniques are known for using additive manufacturing to produce hot-section components. For example, U.S. Patent Application Publication 2011/013592 to Morris et al. describes a process in which a component is built up through repeated cycles of depositing metallic powder followed by laser melting. The laser heat input is sufficient to maintain required solutioning temperatures for a portion of a component, but cannot produce components having a single-crystal microstructure throughout.
Accordingly, there is a need for a process for additive manufacturing of components having a single-crystal microstructure.